problems with citroen Relay panel van starting?

On a few occasions the van has turned o0ver but not started. RAC on the first time said the key was not recognised, changed a fuse and it appeared ok. This time, the RAC tried the fuse again and other things but it still would not start, whilst being towed to the garage the lights seemed to reset and when we had stopped i tried it and it started. At the garage they couldn't test it as it was running.
Can anyone throw any light on this?